OrcaFlex Manual - Orcina

w
Dynamic Analysis
207
Theory, Winch Theory
If Specified Payout is specified (length control mode) for a stage of the simulation, then the unstretched length of
winch wire paid out, L0, is steadily increased or decreased during that stage so that the total change during the stage
is the Value specified. Positive Value means pay out, negative Value means haul in.
The winch wire tension t (which is applied to each point on the wire) is then given by
where
t = K . ε + C . K . dε/dt (2)
K and C are the Wire Stiffness and Wire Damping data values specified on the winch data form
ε = Wire Strain = [ L - L0 ] / L0
dε/dt = Wire Strain Rate = [ dL/dt - dL0/dt ] / L0
L = total length of the winch wire path
Note: The winch wire is not allowed to go into compression, so if the value for t given by equation (2) is
negative then the winch wire is considered to have gone slack and t is set to zero.
If Specified Tension mode is specified for a stage of the simulation, the Value specified in the data is used as a
target nominal constant tension that the winch drive attempts to achieve.
For Simple winches the winch drive is always assumed to achieve the target tension, so the Value specified is used
for the actual winch wire tension.
For Detailed winches the winch drive tries to achieve the target tension by applying a drive force to one side of the
winch Inertia, opposing the wire tension being applied to the other side. The drive force f applied is then given by:
where
if dL0/dt < 0: f(V) = f(0) - Deadband + A.V - C.V 2
if dL0/dt = 0: f(V) = f(0)
if dL0/dt > 0: f(V) = f(0) + Deadband + B.V + D.V 2 (3)
V = rate of payout = dL0/dt
Deadband = the winch drive deadband data item
A, B = the winch drive damping term data items
C, D = the winch drive drag term data items.
f(0) = Value + Stiffness.(L0 - L00)
Value = the nominal constant tension Value given
Stiffness = the winch drive stiffness data item
L00 = original value of L0 at the start of the simulation (set by the static analysis)